1. Field of the Invention
The present invention is in the field of liquid pumps. More particularly, the present invention is in the field of combined axial and centrifugal pumps for pumping liquids. Still more particularly, the present invention relates to a combined axial and centrifugal pump having a centrifugal-flow section and an axial-flow helical inducer section conveying liquid flow to the centrifugal section. The present inventive pump has particular utility for pumping blood with minimal damage to the formed cells and other constituents of the blood.
2. Related Technology
Blood is a complex and delicate fluid. Blood is essentially made up of plasma, which is a pale yellow liquid containing microscopic materials including the formed constituents of the blood. These formed constituents include the red corpuscles (erythrocytes), the white corpuscles (leukocytes), and the platelets (thrombocytes). These and other constituents of the blood, as well as the suspension of the formed constituents of the blood in the plasma, are affected by the manner in which blood is physically handled or treated. Moreover, blood is susceptible to damage from a variety of physical effects. These include depressurization, shock, shear, impact, other forms of physical stress, as well as turbulence, and sudden changes in cross sectional area of a flow path which causes eddy currents, and which may cause small clots to form in the blood.
Further to the above, it is well recognized that the conventional type of positive-displacement roller pump which has many uses in the medical field, and which employs a length of flexible tubing squeezed in progressive sections between a housing and moving rollers is hard on the formed constituents of the blood. These formed blood constituents become caught in the moving nips formed as the rollers move along and squeeze the flexible tubing. When so caught and squeezed, the formed constituent cells are ruptured and destroyed.
During many major surgical procedures, such as open heart surgery, or cardiovascular-respiratory support, the need to pump blood arises. This blood pumping necessity arises in connection with the requirement to move blood for the patient through heart-lung systems which filters the blood, removes impurities, oxygenates the blood, and controls its temperature to a level which reduces the patient's metabolism during the surgery. However, as pointed out above, pumping of blood is a particularly difficult problem. Such is the case because blood is not a simple liquid which can be pumped as though it were water or oil, for example. As explained above, blood contains many cells, such as red and white corpuscles, and other constituents which are living tissues of the body. These living blood tissues are particularly susceptible to damage and destruction by pumping actions which squeeze the blood, as do roller type pumps. Also, pumps which subject the blood to sudden changes in direction or velocity, which excessively stir the blood, which subject the blood to high relative velocities, or which submit the blood to excessive levels of shear, all impose damage on the blood, and impose a trauma on a living tissue of the patient undergoing the surgery or medical procedure.
Many attempts have been made to provide dynamic pumps which avoid the deficiencies of the conventional roller pump and other conventional pumps. One of these conventional blood pumps is known in accord with U.S. Pat. No. Re. 28,742, reissued 23 March 1976, to E. H. Rafferty, et al. The Rafferty reissue patent is believed to teach a dynamic blood pump based on the forced vortex principle. That is, the pump is based on the principle that a spinning chamber forms therein a spinning volume of liquid which is pressurized at its outer periphery and flows radially outwardly. The Rafferty pump defines one or a successive plurality of such spinning chambers, the walls of which may be smooth with the exception of strut members or other such structural features arranged to connect the walls together for rotation in unison, or which may include radially extending and perhaps forwardly or backwardly inclined or swept vanes. In this context, the terms forwardly or backwardly inclined or swept vanes refer to the circumferential direction in which the vanes extend with respect to the normal direction of rotation of such a pump rotor. When so equipped with vanes extending into the pumping chambers from the rotating wall surfaces, the pump configurations of Rafferty are more akin to centrifugal pumps than to strict forced vortex pumps, the latter which rely on viscous coupling between the rotating walls of the pump and the liquid to effect spinning and pressurization of the latter.
As may easily be appreciated, the struts and other structural features of the pumps of Rafferty, and particularly the vanes of these pumps which are of centrifugal configuration may impose severe damage on formed blood cells. That is, the struts are spaced outwardly from the axis of rotation and sweep through the blood with a high relative circumferential velocity. Similarly, the vanes of the centrifugal versions of Rafferty's pumps have edges which may result in abrupt changes in cross sectional area of the blood flow channels, in turbulence and in shock to the formed constituents of the blood.
Another series of dynamic pumps which are based on the forced vortex principle and viscous coupling of the blood with the pumping rotors are seen in U.S. Pat. Nos. 3,864,055; 3,647,324; 3,970,408; 3,957,389; and 4,037,984. Considering particularly the first-listed one of these patents, it is seen that the pumping elements are based on forced vortex principles with the pumping chambers being of disk-like, semi-spherical, conical, or trumpet-shaped configurations. The pumping chambers include a pair of axially spaced apart rotating end wall members, and may include intermediate wall members which are matched in shape to the end wall members.
Experience has shown that with blood pumps made according to the teachings listed immediately above, the pumped blood experiences a higher than preferred level of damage. Also, some of these pump configurations are rather complex and expensive to manufacture. In the present medical environment with concerns about Hepatitis and AIDS mitigating toward a minimization of contact with a patient's blood, and the disposal of blood wetted equipment, the discarding of such pumping devices after a single use constitutes a significant expense.
Still additional conventional dynamic blood pumps are seen in U.S. Pat. No. 4,625,712, issued 2 December 1986; and U.S. Pat. No. 4,846,152, issued 11 July 1989, each having R. K. Wampler as a sole or coinventor. The blood pumps of the Wampler patents are small, high-speed, axial-flow designs of single or multiple stages, and the single stage design includes a slight increase in inner diameter of the flow path so that some centrifugal pumping effect is asserted.
The Wampler pumps have been found by experience to impose severe damage on the formed constituents of blood. It is believed that the high rotational speed which are required for the Wampler pumps to achieve significant volumes of pumped blood against ordinarily head pressures encountered in the use environments of such pumps is a significant factor in the great damage these pumps impose on the pumped blood.
Finally, another conventional blood pump of novel design is shown in U.S. Pat. No. 5,040,944, issued 20 August 1991, to E. P. Cook. The Cook teaching includes a pump with an elongate central ribbon-like member which is helical and stationary. Around this central member rotates an elongate helical rod-like member which has a direction of helix opposite to the central member and which also rotates in this direction.
While the blood pump according to the Cook patent is believed to offer advantages in pumping volume and developed head pressure in comparison to other conventional blood pumps, the rotational speed required of this pump is still much higher than desired. Consequently, the Cook pump also imposes somewhat more damage on the pumped blood than that which is considered minimal and acceptable.